There are various applications in which so-called electrostatic dissipative (ESD) products are desirable. Such products are useful in the manufacturing and use of electronic equipment where sensitive electrical components accumulate static charges which can interfere with their operation or actually cause damage to such components. Similarly in the use or manufacture of electronic equipment, accumulation of static charges on substrate surfaces such as floors, workbenches, cabinets and the like which might be imparted to such equipment in the course of normal operations, can lead to serious problems. Another application is in the use of thermoplastic polymers which can be thermoformed by extrusion-blowing and the like to produce fibers and thin films are sheets which have electrostatic dissipative characteristics.
Various formulations incorporating complexes formed from metal salts in a polymer matrix have been proposed in order to provide electrically conductive surface layers or films which can function to prevent the buildup of static charges on surfaces or dissipate charges transferred from other bodies. The metal ions may be introduced by means of a suitable dopant moiety which imparts the desired conductive characteristics to the polymer matrix. The use of such dopants to introduce trace impurities into polymer matrixes or structures is well known in the art.
An antistatic polymeric composition useful as a floor polish is disclosed in U.S. Pat. No. 4,872,910 to Eshleman et al. Here, various alkali metal or alkali earth metal salts are employed in conjunction with acryloid-type polymer materials in a polyalkylene oxide complex with the metal ions. By way of example, polyethylene oxide or polypropylene oxide can be used to form a complex with various halides, thiocyanates, acetates or nitrates of lithium, sodium or potassium or magnesium, calcium or strontium. Polymers and surfactants can be incorporated into the formulation as well as the alkali metal or alkali earth metal-polyalkylene oxide complex. For example, polymers which can be incorporated into the Eshleman et al formulation include polyurethanes, acrylate copolymers, acrylic acid terpolymers, polyvinyl alcohol, polyethylene glycol, styrene-maleic anhydride copolymers, together with non-ionic surfactants and plasticizers and various other additives. In the Eshleman formulation the preferred metal salt is a lithium salt, specifically, lithium chloride.
Another formulation, in which polymers are treated with an electron donor dopant to provide electrically conductive films, is disclosed in U.S. Pat. No. 4,755,326 to Liepina et al. Here an electride or alkalide, characterized respectively as a salt in which the anion is a trapped electron or an alkali metal anion, incorporating "trapping agent" which may be a podand, cryptand or coronand. Specifically, a crown-ether such as 18-crown-6 is employed. While in the Liepina formulation, the preferred metal for use in the electron donor dopant is cesium, other alkali metals are identified as useful and include lithium, sodium, potassium and rubidium. In addition, lithium may be used in conjunction with cesium. The polymer which is subject to the dopant may be polyactylene or a polyaromatic heterocyclic polymer which in the unmodified (non-doped state) is electrically non-conductive. Polymers disclosed as useful in Liepina include polyquinoxalines, polybenzimidazoles, polybenzoxazoles, polybenzthiazoles, polyoxadiazoles, polybenztriazoles and polysulfodiazoles. Specifically, disclosed are polymer structures derived from Pyrrone I, Pyrrone II, and polyphenylquinoxaline. The Liepina formulation involves a doping procedure which is carried out in a vacuum system, followed by vacuum distillation and drying to effect the curing action, resulting in cured film thickness ranging from 50 to 100 microns and ranging as reported in Table I of the patent from bright purple to black.
U.S. Pat. No. 4,711,742 to Jen et al discloses solutions having as ingredients, film forming homopolymers or copolymers, organic solvents and electron acceptor dopant solutes although electron donor dopants can also be used. Numerous polymers and organic solvents are disclosed in Jen et al as useful in forming the conductive films. Such polymers include various polymers of the teropentacyclic such as furans and thiofurons substituted with alkyl, aryl, or thio-, carboxyl, or sulfonic acid substituted alkyls or aryls or other function groups. Various solvents included sulfones, alkyl alkane sulfonates, flrans, ethers, and aliphatic or aromatic hydrocarbon solvents. A large number of electron acceptor dopants are disclosed including various halogens, chlorates, nitrates, sulfites, tri- and tetra-halides, and perchlorates, including lithium perchlorate.
Other formulations disclosed in U.S. Pat. No. 4,438,251 to Herweh are based upon polyurethane polymers which are modified to incorporate macrocyclic ether moieties into the polymer backbone. Specifically disclosed are polyurethane-based polymers incorporating dibenzyl-substituted crown ethers, such as 18-crown-6, into the polymeric backbone. The resulting polymer products are then doped to provide cationic moieties such as potassium ions in a guest/host complex with the crown ether segment of the polymer chain. By way of example, various diphenyl substituted crown ethers, such as diphenylene 18-crown-6, diphenylene 12-crown-4, and diphenylene 24crown-8, may be employed in reaction with diisocyanates. Specifically disclosed in is a compound such as the reaction products of dibenzo-18-crown-6 diols with methylene bis(4-cyclohexyl isocyanate) followed by doping of the resulting polymer with potassium 7,7,8,8-tetracyanoquinodimethane (TCNQ).
Another approach to alleviate the adverse effects of electrostatic discharge involves the application of a protective coating to a non-conductive substrate to render it conductive or static dissipative as disclosed, for example, in British Patent No. 2148915A to Berbeco. As disclosed in Berbeco, ionic conduction is employed through application of a humidity-dependent coating in which small amounts of moisture are present to allow for the migration of ions and, hence, the overall flow of electrons. Such ionic conductive-based coatings can be characterized as conductive having a resistance of 1-10.sup.5 ohms per square or static dissipative having a specific resistance of 10.sup.5 -10.sup.12 ohms per square. Another system involving the use of static dissipative self-protonating polymeric composition that are humidity-independent is disclosed in U.S. Pat. No. 5,320,780 to Unruh. As disclosed in Unruh, an aromatic-based conductive polymer system can be employed to provide a polymeric formulation which can be applied to a substrate material and which is humidity-independent. As disclosed in the Unruh '780 patent, various polyaniline-based polymers can be employed to provide conductive polymer systems which can be applied to a substrate material to provide an end product having a resistivity within the range of about 10.sup.5 -10.sup.12 ohms/square. Specifically disclosed is a self-protonating conductive polymer which can be a sulfonated or phosphated aromatic unsaturated hydrocarbon which is nitrogen bridged and which is employed in conjunction with an acrylic emulsion and polyethylene emulsions and a glycol ether together with a plasticizer. Specifically disclosed in the '780 patent are various polyanilines and sulfonated or phosphated polyanilines, the former being preferred, resulting in a product which, when applied to a substrate surface, results in a humidity-independent film.